Projection Exposure System, Beam Delivery System and Method of Generating a Beam of Light

ABSTRACT

A beam delivery system of a projection exposure system comprises a laser generating a beam of laser light from a plurality of longitudinal laser modes in a cavity, wherein light generated by a single longitudinal laser mode has an average line width λ lat , wherein the laser light of the beam has, at each of respective lateral positions of the beam, a second line width λ lat  corresponding to lateral laser modes, and wherein the laser light of the beam has, when averaged over a whole cross section thereof, a line width λ b  corresponding to plural lateral laser modes, and wherein λ m &lt;λ lat &lt;λ b , and wherein an optical delay apparatus disposed in the beam provides an optical path difference Δl, wherein 
     
       
         
           
             
               
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     wherein λ 0  is an average wavelength of the light of the first beam of laser light, and Δλ lat  represents the second line width.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of generating a beam of light,a beam delivery system and a projection exposure system for imaging apatterning structure onto a light sensitive substrate.

2. Brief Description of Related Art

Lithographic processes are commonly used in the manufacture ofminiaturized structures, such as integrated circuits, liquid crystalelements, micro-patterned structures and micro-mechanical components.

A projection exposure system used for photolithography generallycomprises a projection optical system for imaging a patterningstructure, commonly referred to as a reticle, onto a substrate, commonlyreferred to as a wafer. The substrate is coated with a photosensitivelayer, commonly referred to as a resist, which is exposed with an imageof the patterning structure using imaging light. The imaging light isgenerated by a beam delivery system illuminating the patterningstructure with the imaging light.

The beam delivery system comprises a laser light source, such as anexcimer laser, for producing the imaging light.

It has been observed that, due to the spatial coherence of the laserlight, interferences of the laser light result in a non-homogeneousintensity of the imaging light in a plane where the patterning structureis disposed. Such non-homogeneous distribution of intensity of light,which is also known as speckle noise, may result in a reduced imagingperformance of the projection exposure system.

SUMMARY OF THE INVENTION

The present invention has been accomplished taking the above problemsinto consideration.

Embodiments of the present invention provide a method of reducing avisibility of speckles in a projected image of a projection exposuresystem.

Other embodiments of the present invention provide a beam deliverysystem providing a beam of light having a reduced coherence as comparedto laser light directly emitted from a laser of the beam deliverysystem.

Further embodiments of the present invention provide a projectionexposure system having an improved imaging performance due to animproved homogeneity of light used for illuminating a patterningstructure to be imaged.

According to an embodiment of the invention, a method of generating abeam of light, comprises: exciting a plurality of longitudinal lasermodes in a cavity of a laser and combining light generated by theplurality of longitudinal laser modes to form a first beam of laserlight; separating the first beam of laser light into at least one firstpartial beam and at least one second partial beam; and combining the atleast one first partial beam and the at least one second partial beam toform a second combined beam of laser light traversing a beam shapingoptics to be incident on an object plane; wherein the separating andcombining includes separating light of the longitudinal laser modes intoat least first light portions and second light portions and differentlymanipulating the separated first and second light portions. Bydifferently manipulating first light portions and second light portions,the first and second light portions will not identically coincide in anobject plane or image plane of a projection optical system, such thatspeckle patterns generated by the first light portions and specklepatterns generated by the second light portions will not identicallycoincide in the object plane or will experience an reduction due to aninterference between the first and second light portions.

According to an embodiment of the invention, a laser generates a firstbeam of laser light, the first beam of laser light is separated into afirst partial beam and a second partial beam, an optical path differenceof the first partial beam is provided relative to the second partialbeam, and the first and second partial beams are then combined to form asecond beam of laser light. The optical path difference Δl is greaterthan about 0.8·λ₀ ²/(2Δλ_(lat)) and less than about 1.8·λ₀ ²/(2Δλ_(lat))wherein λ₀ is an average wavelength of the light generated by the laser,and Δλ_(lat) is a line width of light generated from a single laterallaser mode of the laser.

According to further exemplary embodiments herein, the optical pathdifference Δl is greater than about 0.85·λ₀ ²/(2Δλ_(lat)) and less thanabout 1.5·λ₀ ²/(2Δλ_(lat)), and the optical path difference Δl may begreater than about 0.9·λ₀ ²/(2Δλ_(lat)) and less than about 1.24·λ₀²/(2Δλ_(lat))

According to an exemplary embodiment of the invention, the combining ofthe first partial beam and the second partial beam is performed suchthat cross sections of the first and second partial beams are disposedadjacent to each other within a cross section of the combined secondbeam.

According to a further exemplary embodiment, plural first and secondpartial beams are combined such that their cross sections arealternatingly disposed within the cross section of the combined beam.

According to an exemplary embodiment, a beam path of the first partialbeam is laterally displaced relative to a beam path of the secondpartial beam. Such displacement of the first and second partial beamsrelative to each other may be greater than a tenth of a distancecorresponding to a width of a lateral laser mode across the crosssection of the first beam of laser light, and less than the distancecorresponding to the width of the lateral laser mode.

According to an embodiment of the present invention, a beam deliverysystem comprises a laser for generating a first beam of laser light andan optical delay apparatus disposed in a beam path of the first beam oflaser light, wherein the optical delay apparatus is configured toprovide an optical path difference of a first partial beam of the firstbeam of laser light relative to a second partial beam of the first beamof laser light, wherein the optical path difference is greater thanabout 0.8·λ₀ ²/(2Δλ_(lat)) and less than about 1.8·λ₀ ²/(2Δλ_(lat)).

According to an exemplary embodiment, the optical delay apparatuscomprises a stack of a plurality of first plates of a transparentmaterial, wherein the stack is disposed in the beam path of the firstbeam of laser light such that plural first partial beams of the firstbeam of laser light traverse the first plates and that plural secondpartial beams traverse spaces between adjacent first plates.

According to a further embodiment of the present invention, a beamdelivery system comprises a laser for generating a first beam of laserlight from a plurality of longitudinal laser modes in a cavity of thelaser; and an optical delay apparatus disposed in a beam path of thefirst beam of laser light, wherein the optical delay apparatus isconfigured to provide an optical path difference of at least one firstpartial beam of the first beam of laser light relative to at least onesecond partial beam of the first beam of laser light; and wherein theoptical delay apparatus comprises a stack of a plurality of first platesof a transparent material disposed at a distance from each other,wherein the first plates are traversed by beam paths of plural firstpartial beams and spaces between adjacent first plates are traversed bybeam paths of plural second partial beams.

In this embodiment, the optical path difference can be greater than1.8·λ²/(2Δλ_(lat)).

According to an exemplary embodiment, the first plates are each orientedsubstantially parallel to a direction of the light traversing theoptical delay apparatus.

According to a further exemplary embodiment, second plates oftransparent material are sandwiched between adjacent first plates. Alength of the first plates and/or a refractive index of the material ofthe first plates differs from a length of the second plates and arefractive index of the material of the second plates, respectively.

According to an exemplary embodiment, the optical delay apparatuscomprises a third plate of transparent material disposed in the beampath of the first beam of laser light such that surfaces of the thirdplate are oriented transversely to the direction of the first beam oflaser light traversing the third plate. The first partial beam directlytraverses the third plate, and the second partial beam is two or moretimes internally reflected from surfaces of the third plate to becombined with the first partial beam.

According to an exemplary embodiment, cross sections of the first beamof laser light immediately upstream of the optical delay apparatus andof the second beam of laser light immediately downstream of the opticaldelay apparatus are substantially the same.

According to a further embodiment of the present invention, a beamdelivery system comprises a laser for generating a beam of laser lightfrom a plurality of longitudinal laser modes in a cavity of the laser;an optical delay apparatus disposed in a beam path of the beam of laserlight, wherein the optical delay apparatus comprises plural reflectivesurfaces arranged such that the beam path comprises a closed loop; atleast one phase changing element disposed in the beam path of the closedloop, wherein the phase changing element comprises a structured phasechanging surface having a plurality of projections and indentations ofamplitudes of more than 100 nm.

According to an exemplary embodiment herein, lateral extensions of theprojections and indentations are smaller than lateral extensions ofcoherent portions of laser light interacting with the phase changingsurface and originating from single longitudinal laser modes in thecavity. This means that the lateral extensions of the projections andindentations are smaller than lateral extensions of coherent coherencecells of the laser light of the beam at a location of the phase changingelement.

Thus, light generated by one longitudinal laser mode interacts withplural different projections and/or indentations such that wave frontsof the laser modes are changed by each interaction of the laser lightwith the phase changing surface occurring in each traversal of theclosed loop.

According to a still further embodiment of the invention, a beamdelivery system comprises a laser for generating a beam of laser lightfrom a plurality of longitudinal laser modes in a cavity of the laser;an optical delay apparatus disposed in a beam path of the beam of laserlight, wherein the optical delay apparatus comprises plural reflectivesurfaces arranged such that the beam path comprises a closed loop; atleast one phase changing element comprising a phase changing surfacedisposed in the beam path of the closed loop; and a surface wavegenerator for generating surface acoustic waves propagating across asurface portion of the phase changing surface exposed to the beam oflaser light traversing the optical delay apparatus.

The generated surface acoustic waves have an effect of generating astructured phase changing surface such that different portions of laserlight generated by one single longitudinal laser mode experiencedifferent phase changes when traversing the phase changing surface.

In the above embodiments, the phase changing element has an effect ofartificially increasing an effective number of modes or effective numberof coherence cells of the light of a laser pulse. Thus, an increasednumber of independently uncorrelated speckle patterns is formed, whereinthe increased number of speckle patterns superimposed in the image planeresults in a reduced observable variation of light intensity in theimage plane.

The laser may comprise an excimer laser, such as a KrF laser, an ArFlaser and an F₂ laser.

According to a further exemplary embodiment, the laser comprises a linenarrowing module, having optical elements, such as a prism and areflective grating.

According to further exemplary embodiments, the beam delivery system maycomprise further optical elements, such as a dispersion plate, adiffractive optical element, a refractive optical element, and others.

According to an embodiment of the present invention, a projectionexposure system comprises a projection optical system for imaging anobject plane into an image plane, a first mount for mounting a patteringstructure in a region of the object plane, and a second mount formounting a substrate in a region of the image plane of the projectionoptical system. The projection exposure system further comprises a beamdelivery system as illustrated above for generating imaging light forillumination of the object plane.

According to a further embodiment of the present invention, a projectionexposure system comprises a beam delivery system including a laser forgenerating a beam of laser light from a plurality of longitudinal lasermodes in a cavity of the laser, and an optical delay apparatus disposedin a beam path of the beam of laser light; and a projection opticalsystem for imaging a patterning structure disposed in an object plane ofthe projection optical system into an image plane thereof; wherein abeam of laser light delivered by the beam delivery system to illuminatethe patterning structure disposed in the object plane has aspeckle-generated intensity variation of less than 2% across the objectplane.

Such reduced speckle contrast in the light illuminating the patterningstructure has a particular advantage in generating a substantiallyuniform light intensity in the object plane of the projection opticalsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing as well as other advantageous features of the inventionwill be more apparent from the following detailed description ofexemplary embodiments of the invention with reference to theaccompanying drawings. It is noted that not all possible embodiments ofthe present invention necessarily exhibit each and every, or any, of theadvantages identified herein.

FIG. 1 is a schematic illustration of a projection exposure systemaccording to an embodiment of the invention;

FIG. 2 is a schematic illustration of a laser of a beam delivery systemof the projection exposure system shown in FIG. 1;

FIG. 3 is a graph illustrating line widths of laser light generated bythe laser shown in FIG. 2;

FIG. 4 is a schematic illustration of longitudinal and lateral lasermodes of the laser shown in FIG. 2;

FIG. 5 is a perspective view illustrating an optical delay apparatus ofthe beam delivery system of the projection exposure system shown in FIG.1;

FIG. 6 is a graph illustrating an optical path difference generated bythe optical delay apparatus shown in FIG. 5;

FIG. 7 is a sectional view of a further embodiment of an optical delayapparatus which may be used in the beam delivery system of theprojection optical system shown in FIG. 1;

FIG. 8 is a sectional view of a further embodiment of an optical delayapparatus which may be used in the beam delivery system of theprojection optical system shown in FIG. 1;

FIG. 9 is a sectional view of a further embodiment of an optical delayapparatus which may be used in the beam delivery system of theprojection optical system shown in FIG. 1;

FIG. 10 is a sectional view of a phase changing element of the opticaldelay apparatus shown in FIG. 9;

FIG. 11 is a sectional view of a further example of a phase changingelement which may be used in the optical delay apparatus shown in FIG.9;

FIG. 12 is an elevational view of a further example of a phase changingelement which may be used in the optical delay apparatus shown in FIG.9;

FIG. 13 is a perspective view of a further embodiment of an opticaldelay apparatus which may be used in the beam delivery system of theprojection optical system shown in FIG. 1;

FIG. 14 is a schematic illustration of a projection exposure systemaccording to a further embodiment of the invention; and

FIG. 15 is a perspective view of a further embodiment of an opticaldelay apparatus which may be used in the beam delivery system of theprojection optical system shown in FIG. 1 or 2.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments described below, components that are alikein function and structure are designated as far as possible by alikereference numerals. Therefore, to understand the features of theindividual components of a specific embodiment, the descriptions ofother embodiments and of the summary of the invention should be referredto.

FIG. 1 schematically illustrates a projection exposure system 1. Theprojection exposure system 1 comprises a projection optical system 3comprising a plurality of lenses or mirrors for imaging an object plane5 of the projection optical system 3 onto an image plane 7 of theprojection optical system 3. A reticle 9 is mounted by a reticle stage11 such that a pattering structure provided by the reticle 9 is disposedin the object plane 5. A wafer 13 is mounted on a wafer stage 15 suchthat a light sensitive resist provided on a surface of the wafer 13 isdisposed in the image plane 7. The patterning structure of the reticle 9is illuminated by a beam 17 of imaging light generated by a beamdelivery system 21.

The beam delivery system 21 comprises a laser light source 23 which is,in the present embodiment, an excimer laser, such as a KrF laser, a ArFlaser and a F₂ laser. A beam 25 of laser light generated by the laserlight source 23 traverses an optical delay apparatus 27, which will beillustrated in more detail below, for reducing a coherence of the laserlight. The beam 25 then traverses a beam expander 29, which may comprisea refractive optical element, a lens system 31, a refractive opticalelement 33, such as a fly eye element, a further lens system 35, adiffractive plate 37, a beam homogenizing apparatus 39, such as a glassrod, and a further lens system 41 to be reflected from a mirror 43 to beincident on the reticle 9. The optical elements 29, 31, 33, 35, 37, 39and 41 are disposed such that a light intensity of the light of theilluminating beam 17 is substantially constant across an illuminatedregion of the reticle 9 and has a desired angular distribution relativeto the object plane 5.

The optical elements 29 to 41 illustrated so far may be of aconventional arrangement for shaping the illuminating beam 17. Furtherconventional arrangements of beam delivery systems are known, forexample, from U.S. Pat. No. 6,285,443 B1, U.S. Pat. No. 5,926,257 andU.S. Pat. No. 5,710,620, the contents of which are incorporated hereinby reference.

FIG. 2 is a schematic illustration of the excimer laser light source 23.The laser light source 23 comprises a gas chamber 51 containing a gaswhich is excited by high voltage pulses to produce excimer moleculesemitting ultraviolet light.

The laser light source 23 further comprises a semi-transparent mirror 53which forms an exit window of the laser which is traversed by thegenerated beam 25. The laser 23 further comprises a line narrowingmodule 55, comprising an aperture 57, two prisms 59 and a reflectivegrating 61. A further beam defining aperture 63 is disposed between thegas chamber 51 and the exit window 53.

The laser 23 may generate pulses of laser light at a repetition rate of,for example, 4,000 Hz to 6,000 Hz, each pulse having a duration of about20 ns to about 150 ns. For example, 40 pulses are used for exposing eachpattern onto the wafer. The laser 23 is a multi-mode laser supporting aplurality of lateral and longitudinal laser modes. An average wavelengthof the light 25 emitted from the laser is λ₀. Due to the line narrowingmodule 55, the distribution of wavelengths about the average wavelengthλ₀ is a relatively narrow line width Δλ_(b) as illustrated in FIG. 3which shows a line 65 representing a distribution of light intensity Iin dependence of the wavelength λ. The light 25 emitted from the laser23 originates from plural longitudinal and lateral laser modes, eachhaving a line width smaller than the line width Δλ_(b) of the light ofbeam 25. FIG. 3 shows a line 67 illustrating a spectral intensitydistribution of an exemplary laser mode having a line width Δλ_(m).

FIG. 4 is a schematic illustration of a light pulse 71 generated bylaser 23. The light pulse 71 is composed of light generated from aplurality of longitudinal and lateral laser modes within chamber 51,resulting in limited volumes 73 of coherent light. In this simplifiedrepresentation, each volume 73 of coherent light has an extensionl_(c)(long) in the longitudinal direction and an extension l_(c)(lat) inboth lateral directions. Reference numeral 75 in FIG. 4 indicates pluralcoherence volumes generated by different lateral modes at a same time,and reference numeral 76 indicates coherence volumes generated bydifferent longitudinal modes at a same lateral position. The set oflongitudinal modes 76 contributing to light emitted at a particularlateral position is also referred to as a lateral mode.

It should be noted that the illustration of FIG. 4 is very schematic andonly for illustrative purposes. In reality, a coherence cell does nothave a cubic shape as suggested by FIG. 4. Further, different coherencevolumes will overlap both in longitudinal and lateral directions.

Light within a single coherence volume 73 is coherent light, such thatan interference pattern may be formed from light originating from asingle coherence cell 73. However, light from one coherence cellsuperimposed with light from a different coherence cell will notgenerate an interference pattern since a coherence condition is notfulfilled between different cells.

The light of the exemplary coherence cell 73 indicated as a hatched cellin FIG. 4 is assumed to have the spectral density as illustrated by line67 in FIG. 3 at a peak wavelength λ₁. The light from other coherencecells has different peak wavelengths and may have slightly differentline widths such that the combined light of all coherence cells has aspectral distribution as indicated by line 65 in FIG. 3.

Due to the geometry of the laser cavity 51 and a dispersion of the linenarrowing module 55, the average wavelength of the light emitted by thelaser will change across the cross section of the beam 25. It appearsthat a spectral distribution of the laser light generated by a singlelateral mode formed of plural longitudinal modes arranged in a lineparallel to the direction of the beam is narrower than the spectraldistribution of the whole beam and broader than the spectraldistribution of the longitudinal laser mode. Line 66 in FIG. 3 indicatesa spectral distribution of an exemplary lateral laser mode.

The following Table 1 illustrates data of an exemplary KrF laser and anexemplary ArF laser.

TABLE 1 Excimer KrF ArF λ₀ [nm] 248 193 Δλ_(b) [pm] 0.5 0.5 l_(b) [cm]6.15 3.72 N_(long) 80-500 80-500 N_(lat) 100 100 N_(tot) 8,000-50,0008,000-50,000 Δλ_(lat) [pm] 0.25 0.25 l_(lat) [cm] 12.3 7.45 Δλ_(m) [pm]0.125 0.125 l_(m) [cm] 24.6 14.9λ₀ indicates the peak wavelength, Δλ_(b) the line width of the laserlight generated from the multitude of laser modes forming the beam,N_(long) indicates a number of longitudinal modes, N_(lat) a number oflateral modes and N_(tot)=N_(long)·N_(lat) a resulting total number oflaser modes contributing to one pulse.

At each lateral position of the beam, the light is generated from alateral mode comprising plural longitudinal modes. A line width of onesingle longitudinal mode is indicated by Δλ_(m), and the resulting linewidth of a lateral mode at a particular lateral position is indicated byΔλ_(lat).

l_(m) indicates a coherence length of the light from one single lasermode calculated by the formula l_(m)=λ₀ ²/2·Δλ_(m).

Table 1 further indicates comparative expressions l_(lat)=λ₀²/2·Δλ_(lat), and l_(b)=λ₀ ²/2·Δλ_(b).

The light from one single coherence cell 73 traverses the beam deliverysystem 21 to be incident on the object plane 5. At each location of theobject plane 5, the incident light is composed of light rays havingtraversed different paths through the beam delivery system and havingexperienced slightly different optical path lengths accordingly. Thecoherent light from one single coherence cell 73 may thus generate aninterference pattern, such as a speckle pattern, in the object plane 5.The light intensity will be modulated across the object plane, wherein aspeckle contrast may be as high as 100% which means that constructiveinterference will take place at some locations and completelydestructive interference may take place at other locations.

Since light originating from different coherence cells will notinterfere with each other, each coherence cell will contribute to anindependent speckle pattern in the object plane. Such independentpatterns will result in an averaging of the light intensities in theobject plane such that the intensity modulation is reduced by averagingby a factor 1/√{square root over (N_(tot))} as compared to themodulation generated by the light of one single coherence cell.

Even with such averaging, the light intensity distribution may be notsufficiently constant in the object plane 5. Therefore, the opticaldelay apparatus is disposed in the beam path of the imaging light.

Moreover, it should be noted that, depending on the geometry of the beamdelivery system, it is possible that different longitudinal modes maygenerate the same speckle patterns in the object plane. In suchsituations, the number N of modes contributing to the averaging isdetermined by the lower number N_(lat) of longitudinal modes rather thanthe total number N_(tot) of modes supported by the laser.

The optical delay apparatus 27 has a function of reducing the coherenceof the illuminating light and has a configuration as shown in FIG. 5.The optical delay apparatus 27 comprises a plurality of glass plates ofa thickness d₁ which are spaced at a distance d₂ from each other. Theglass plates 81 are mounted as a stack and fixed by two frames 83 (onlyone frame is shown in FIG. 5) engaging the plates 81 at lateral sides 85thereof. A broken line 87 illustrates a cross section of beam 25incident on front surfaces 89 of plates 81. Flat main surfaces 91 of theplates 81 extend over a length L in a direction parallel to the beam 25.

The beam 25 incident on the stack of plates 81 is separated into firstpartial beams traversing the plates 81 and second partial beamstraversing the spaces between adjacent plates 81. The first partialbeams traversing the plates 81 experience an optical delay or opticalpath length difference Δl=(n−1)·L relative to the second partial beamstraversing the spaces between adjacent plates 81 assuming a refractiveindex equal to 1 for the medium between adjacent plates. The length L ischosen such that Δl=λ₀ ²/2·Δλ_(lat). Further, a pitch d₁+d₂ of the stackis chosen such that it is about equal to or less than a lateralextension of a lateral laser mode in the incident beam 25. Assuming asquare shaped beam cross section, the lateral extension of a laterallaser mode is about the diameter of the beam divided by √{square rootover (N_(lat))}, wherein √{square root over (N_(lat))} is the number oflateral modes of the beam. Thus, the coherent light from one singlelateral laser mode is separated into at least one first partial beamexperiencing the optical delay and at least one second partial beamtraversing the optical delay apparatus 27 without delay.

This is further illustrated in FIG. 6, in which a first line 95represents a temporal intensity distribution of the non-delayed firstpartial beam and a second line 97 indicates a temporal intensitydistribution of the delayed first partial beam.

It is apparent that each of the light pulses 95 and 97 may form aspeckle pattern in the object plane 5, such that the delay apparatus 27has a first effect of doubling the number of light modes capable offorming an interference pattern in the object plane. This first effectof the delay apparatus 27 reduces the intensity modulation in the objectplane 5 by a factor of 1/√{square root over (2)}.

Further, due to the temporal overlap of pulses 95 and 97, light of thepulse 95 may interfere with the coherent light of the pulse 97. However,the light traversing the plates 81 not only experiences a delay byl_(lat) relative to the light traversing the spaces between plates 81,the light traversing the plates further experiences a phase shiftrelative to the light not traversing the plates 81. This phase shift iswithin a range from zero to 2π, depending on the length L and thewavelength λ of the light. Since the wavelength λ of the laser modes hasa random distribution about the peak wavelength λ₀, the resulting phaseshifts which the light from the various laser modes experiences from theoptical delay apparatus 27 will also have a random distribution.Therefore, a portion of the light intensities of pulses 95 and 97indicated as a hatched portion in FIG. 6 generates an interferencepattern in the object plane 5 which is different for each coherence cellsuch that a random averaging takes place. Thus, a second effect of theoptical delay apparatus 27 is to introduce random phase shifts betweenfirst and second partial beams for further reducing the intensitymodulation in the object plane.

With the optical delay apparatus as illustrated above, it is possible toreduce a coherence of the laser light to such an extent that theintensity modulation in the object plane, and, thus, in an image plane,of the projection exposure system due to speckles is as low as 1%.

According to a further embodiment, an optical delay apparatus as shownin FIG. 5 is disposed in the beam 25, wherein the length L of the plates81 is increased to generate substantially greater optical delays oroptical path length differences ΔL which are greater than Δλ₀²/2·Δλ_(1at), such that a substantial overlap between lines 95 and 97 inFIG. 6 does no longer occur. Even without such overlap, this embodimentsignificantly contributes to reducing a speckle contrast in the objectplane since the number of laser modes which will not interfere with eachother is increased.

According to a still further embodiment, plural optical delayapparatuses of the type illustrated in FIG. 5 are disposed in the beampath of beam 25. For example, a first optical delay apparatus 27 may bedisposed upstream of a second optical delay apparatus 27 in the beam 25.The first and second delay apparatuses may have different orientationsof their plates 81 relative to the beam 25. For example, the first delayapparatus may have its plates 81 oriented in a horizontal direction asillustrated in FIG. 5, and the second delay apparatus may have its plate81 oriented in a vertical direction.

According to a further example, both the first and second optical delayapparatuses have their plates oriented in a same direction, wherein theplates of the second delay apparatus are laterally displaced relative tothe plates of the first optical delay apparatus. Thus, portions of thelaser light of the beam will traverse only plates of the first opticaldelay apparatus, other portions of the beam will traverse only plates ofthe second optical delay apparatus, other portions will traverse platesof both the first and second optical delay apparatus, and still otherportions of the beam will traverse none of the plates of the first andsecond optical delay apparatuses.

Still further, it is possible that the first and second optical delayapparatuses have plates of different lengths L.

FIG. 7 illustrates a further embodiment of an optical delay apparatus 27a which may be used in the beam delivery system.

The optical delay apparatus 27 a has a similar configuration as thatshown in FIG. 5, such that a plurality of parallel glass plates 81 ahaving a thickness d₁ disposed at a distance d₂ from each other. Theplates 81 a have a length L₁ in a direction of an incident beam 25 a.Further, glass plates 82 having a thickness d₂ are sandwiched betweenadjacent plates 81, wherein front surfaces of the plates 82 areregistered with front surfaces 89 a of the plates 81 a. The plates 82have a length L₂ in the direction of the beam 25 a which is less thanthe length L₁ of the plates 81 a. The lengths L₁ and L₂ are dimensionedsuch that the desired optical delay l_(m) is generated, such that L₁ andL₂ fulfill the relation:

l _(m) =L ₁(n ₁−1)−L ₂(n ₂−1),

wherein

-   n₁ is a refractive index of the material of the plates 81 a,-   n₂ is a refractive index of the material of plates 82, and    wherein a refractive index of the gas or vacuum disposed in the void    spaces between adjacent plates 81 is assumed to be l for simplicity.

Further, the material of plates 82 may have a higher extinction due tosuch as absorption and scattering for the laser light as compared to theextinction of the material of plates 81 a. The lengths L₁ and L₂ arefurther determined such that both first partial beams traversing theplates 81 a and second partial beams traversing the plates 82 experiencea substantially same extinction when traversing the optical delayapparatus 27 a.

FIG. 8 illustrates a further embodiment of an optical delay apparatus 27b. The optical delay apparatus 27 b comprises a glass plate 101 having afirst main surface 103 and a second main surface 105 parallel to surface103. Surface 103 is oriented transversely in a beam 25 b of incidentlaser light, wherein a surface normal of surface 103 is oriented underan angle α relative to the direction of the incident beam 25 b. Thesurface 105 is a semi-transparent surface separating the incident beam25 b into a first beam 26 directly traversing the plate 101, and asecond partial beam 28 which is reflected from surface 105 andthereafter reflected from surface 103 and then traverses surface 105 tobe superimposed with beam 26. A thickness d of the plate 101 is chosensuch that the partial beam 28 experiences a delay Δl=λ₀ ²/(2Δλ_(lat))relative to partial beam 26:

${d = \frac{\Delta \; l}{2n}},$

whereinn is a refractive index of a material of plate 101.

Due to the angle α, the partial beam 28 is laterally displaced by anamount δD relative to the partial beam 26. Such displacement furthercontributes to reducing the modulation of an interference patterngenerated by light of partial beam 28 interfering with light of partialbeam 26.

The displacement δD is advantageously determined based on a lateralextension of a lateral mode. According to a first example the angle αfulfills the relation

${{0.5 \cdot \arctan}\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}} < \alpha < {{5 \cdot \arctan}{\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}.}}$

According to further examples, the angle α fulfills the relation

${{{0.7 \cdot \arctan}\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}} < \alpha < {{2 \cdot \arctan}\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}}},{or}$${{0.8 \cdot \arctan}\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}} < \alpha < {{1.5 \cdot \arctan}{\frac{D_{1}}{d \cdot \sqrt{N_{lat}}}.}}$

Compared to a diameter D₁ of the incident beam 25 b, a diameter D₂ ofcombined partial beams 26, 28 is increased only by the small value δD.

FIG. 9 shows a further embodiment of an optical delay apparatus 27 c.The optical delay apparatus 27 c comprises a prism 111 having fivesurfaces 113, 114, 115, 116 and 117.

Surface 113 is a semitransparent surface separating an incident beam 25c into a first partial beam 26 c directly reflected from the surface 113of the prism 111, and a second partial beam 30 which is refracted at thesurface 113 and enters the bulk material of the prism 111. Beam 30 issubsequently reflected from surfaces 114, 115, 116 and 117 of the prism111 by internal reflection, and is then again incident on surface 113from the interior of the prism 111. A portion of that beam traversessurface 113 and coincides as a delayed beam 28 c with beam 26 c directlyreflected at surface 113. A beam path of beam 30 forms a closed loopwithin prism 111.

The prism 111 may be made of a CaF₂ material having a crystalorientation such that a (100) crystal plane is oriented under an angle φof 45° relative to the surface 113. Such crystal orientation has anadvantage in that an intrinsic birefringence of the material has areduced effect on the beam 30 traversing the material. If the light ofincident beam 25 c is polarized by a polarizer 121, such as a half waveplate 121, the delayed beam 28 c has a substantially same polarizationas the directly reflected beam 26 c.

A further plate like phase changing element 101 c is disposed in thebeam path of beam 30 traversing the prism 111 such that beam 30 issubstantially orthogonally incident on a surface 103 c of plate 101 c.

FIG. 10 schematically shows a portion of an enlarged section of plate101 c. Structured surface 103 c of plate 101 c is a stepped surface suchthat projections 131 and indentation 133 are formed and a thickness ofthe plate 101 c varies across the surface. A minimum thickness is b anda maximum thickness is b+a. Step portions of equal thickness have alateral dimension of s. Both the height a of the steps and the width sof the step portions varies across the surface of plate 101 c. In theillustrated example, the width s is within a range from 0,1 to 5,0 timesa lateral extension l_(c(lat)) of a lateral laser mode in the beam 30traversing the plate 101 c. The maximum height a of the stepped portionscan amount to some plural wavelengths of the laser light. For example,the value of the height a may be in a range from 200 to 500 nm or ashigh as some μm. The projections 131 and indentations 133 can bemanufactured by lithographic methods, for example.

Further, the distribution of the individual heights of the steppedportion and their widths s can have a random distribution such that alsothe phase changing effect of plate 101 c is a random effect across thecross section of beam 28 c.

The structured surface 103 c has an effect that wavefronts of the laserlight traversing the surface 103 c experience minute deformationsresulting in minute changes of propagation directions of the lighthaving traversed the surface 103 c. If this light traverses the surface113 to coincide with the beam 26 c directly reflected from the surface113, it will not be exactly coincident with the directly reflectedlight. Thus, the combined beams 26, 28 include light generated by samelongitudinal laser modes but propagating in slightly differentdirections. This results in different speckle patterns in the objectplane disposed downstream of the optical delay apparatus. Due to theenlarged number of different speckle patterns, the uniformity of thelight distribution in the object plane will be significantly increased.

Moreover, since a portion of the light of the beam 30 having traversedthe structured surface 103 c a first time will be reflected from thesurface 113 and traverse the structured surface 103 c a second time.This results in a further phase changing effect on that light. A portionof that light will traverse the semitransparent surface 113 and becombined with beam 26 c, and a further portion of that light will bereflected from surface 113 c and experience still further phase changingeffects by traversing the structured surface 103 c, and so on.

FIG. 11 schematically shows a further example of a plate 101 d having astructured phase changing surface 103 d. The structured surface 103 dincludes a plurality of projections 131 d and indentations 133 d. Theprojections and indentations 131 d, 133 d form a plurality of smallprisms on the plate 101 d which will cause wavefront deviations of thelaser light traversing the surface 103 d. A characteristic dimension orextension of the prisms in a lateral direction on the surface is lessthan, for example five to ten times less than, a lateral extensionLc(lat) of a portion of laser light originating from a same longitudinallaser mode. Inclination angles ε of surface portions of the prismsrelative to a main surface direction of the surface 103 d may randomlyvary from prism to prism. Still further amplitudes or height differencesbetween projections 131 d and indentations 133 d may randomly vary fromprism to prism.

In the embodiment shown in FIG. 9, the phase changing surface 103 c istraversed by the beam 30. It is, however, possible that the structuredphase changing surface is used as one of the reflecting surfaces 113,114, 115, 116 and 117 such that the beam interacting with the structuredsurface is reflected there from.

FIG. 15 is a perspective view schematically illustrating a furtherembodiment of an optical delay apparatus 27 d comprising a prism forproviding a closed-loop optical beam path similar to that shown in FIG.9. In this embodiment, a structured phase changing surface 103 d isprovided on a reflecting surface 114 d of the prism. A beam of lighttraversing the closed loop is reflected from the structured phasechanging surface 103 d by internal reflection. The surface 103 d isstructured by a plurality of projections 131 d and indentations 133 dforming prisms of random sizes and surface orientations. Therepresentation in FIG. 15 of the projections 131 d and indentations 133d is exaggerated with respect to a size of the projections 131 d andindentations 133 d. In practice, the projections 131 d and indentations133 d are of a small size with characteristic lateral extensions whichare less that lateral extensions of coherence cells of the laser lightincident on and reflected from the structured surface.

FIG. 12 is an elevational view of a reflective phase changing surfacewherein the structure of the reflective surface is generated by surfaceacoustic waves. For this purpose, a surface wave generator 141 includinga plurality of interdigital electrodes 143 is provided on the surface103 e and connected to a high-frequency generator 145. The substratematerial of the plate 101 e providing the surface 103 e is made of apiezoelectric material such that a high-frequency voltage generated bythe high-frequency generator 145 produces surface acoustic wavespropagating in a direction 147 across surface 103 e. Broken lines 149 inFIG. 12 illustrate wavefronts of the surface acoustic waves, and arectangle 151 in FIG. 12 illustrates a portion of surface 103 e in whichthe beam 30 is incident on the surface 103 e and may interact with thesurface acoustic waves 149.

FIG. 13 is a perspective view of a further optical delay apparatussimilar to that shown in FIG. 9. The optical delay apparatus 27 f shownin FIG. 13 differs from that shown in FIG. 9 in that reflective surfaces114 f, 115 f, and 116 f are provided by mirrors 114 f, 115 f, 116 frather than internal reflection surface provided on a prism. Asemitransparent surface 113 f is provided as a beam splitter on whichbeam 25 f is incident, wherein a portion 26 f of that beam traverses thebeam splitter 113 f and a portion 30 f is reflected from the beamsplitter 113 f. Subsequent reflections from reflective surfaces 114 f,115 f and 116 f provide a closed loop beam path of beam 30 f such thatthe beam 30 f is again incident on the beam splitter 13 f. A portion ofthat beam is reflected from the beam splitter and coincides as beam 28 fwith beam 26 f, whereas another portion of the beam traverses the beamsplitter 113 f and traverses the closed loop a second time or a greaternumber of times. Any of the reflective surfaces 114 f, 115 f and 116 fmay be formed as a structured surface of the type illustrated withreference to FIGS. 10, 11 and 12 above. Still further, a plate carryingsuch structured surface can be disposed between any of mirrors 114 f,115 f and 116 f to generate phase changes when the beam 30 f traversessuch plate.

FIG. 14 is a schematic illustration of a further example of a projectionexposure system 1 g in which the optical delay apparatus as illustratedabove can be incorporated. The projection exposure system 1 g comprisesa beam delivery system 21 g for illuminating an object plane 5 g of aprojection exposure system 3 g such that the object plane 5 g is imagedonto an image plane 7 g.

The beam delivery system 21 g comprises a laser light source 23 g, suchas an excimer laser. The laser 23 g may include a beam expanding opticssuch that a beam 25 g emitted from the light source 23 g is already anexpanded beam. The beam 25 g traverses an optical delay apparatus 27 gof the type illustrated above for reducing a coherence of the laserlight to reduce a speckle contrast generated in the object plane 5 g andin the image plane 7 g, accordingly.

The laser light having traversed the optical delay apparatus 27 gtraverses a pupil shaping diffuser 32 which may be provided by adiffraction grating, such as a computer generated hologram (CGH) todefine a shape and light distribution in a pupil plane 171 generated bya condenser lens 164.

A flies eye integrator 33 g is disposed in the pupil plane 171. Thelight having traversed the flies eye integrator 33 g traverses a lens orlens system 163 such that a field plane 157 is provided downstream oflens 163. Field plane 157 is imaged onto the object plane 5 g of theprojection optical system 3 g by a reticle masking lens system 167, 169.A field stop 153 is disposed in field plane 157 for defining thatportion of the object plane 5 g which is illuminated with the laserlight. Since the optical delay apparatus 27 g of the type as illustratedabove is disposed in the beam path of the laser light, a specklecontrast generated in the object plane 5 g is effectively reduced to anamount which may be less than 2% or 1%.

According to further embodiments, an optical delay apparatus as shown inany of FIGS. 5, 7 and 8 can be disposed in a beam path of a type shownin FIG. 9, i.e. disposed in a closed loop path formed by pluralreflective surfaces. It should be noted that the number of fivereflections illustrated in combination with the prism 111 in FIG. 9 isonly an exemplary number. It is also possible to use a lower number ofreflections, such as three or four reflections, or a higher number ofmore than five reflections by suitably adjusting the relative angles ofthe reflecting surfaces. Still further, it is also possible to disposethe plate like phase changing element shown in FIG. 10 in the beam pathof the beam delivery system outside of a closed loop. For example, theplate like phase changing element shown in FIG. 10 may be disposed asthe optical delay apparatus 27 in the beam delivery system shown in FIG.1.

It is envisaged to combine each of the above illustrated embodimentswith any other of the above illustrated embodiments such that a combinedembodiment may comprise one or more features from one of the aboveillustrated embodiments and one or more features of another of the aboveillustrated embodiments.

While the invention has been described with respect to certain exemplaryembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, the exemplary embodiments of the invention set forth hereinare intended to be illustrative and not limiting in any way. Variouschanges may be made without departing from the spirit and scope of thepresent invention as defined in the following claims.

1. (canceled)
 2. A beam delivery system, comprising: a laser forgenerating a beam of laser light from a plurality of longitudinal lasermodes in a cavity of the laser; an optical delay apparatus disposed in abeam path of the beam of laser light, wherein the optical delayapparatus comprises plural reflective surfaces arranged such that thebeam path comprises a closed loop; at least one phase changing elementdisposed in the beam path of the closed loop, wherein the phase changingelement comprises a structured phase changing surface having a pluralityof projections and indentations of amplitudes of more than 100 nm, andwherein at least one of widths and heights of the projections andindentations have a random distribution.
 3. The beam delivery systemaccording to claim 2, wherein lateral extensions of the projections andindentations are smaller than lateral extensions of laser light portionsinteracting with the phase changing surface and originating from singlelongitudinal laser modes in the cavity.
 4. The beam delivery systemaccording to claim 2, wherein lateral extensions of the projections andindentations are smaller than lateral extensions of coherence cells ofthe laser light at a location of the phase changing surface.
 5. The beamdelivery system according to claim 2, wherein the projections andindentations of the phase changing surface have a stepped configuration.6. The beam delivery system according to claim 5, wherein theprojections and indentations of the phase changing surface each includesurface portions oriented substantially parallel to a main surfacedirection of the phase changing surface.
 7. A beam delivery system,comprising: a laser for generating a beam of laser light from aplurality of longitudinal laser modes in a cavity of the laser; anoptical delay apparatus disposed in a beam path of the beam of laserlight, wherein the optical delay apparatus comprises plural reflectivesurfaces arranged such that the beam path comprises a closed loop; atleast one phase changing element disposed in the beam path of the closedloop, wherein the phase changing element comprises a structured phasechanging surface having a plurality of projections and indentations ofamplitudes of more than 100 nm, and wherein the projections andindentations of the phase changing surface have a wedge configuration.8. The beam delivery system according to claim 7, wherein theprojections and indentations of the phase changing surface each includesurface portions inclined relative to a main surface direction of thephase changing surface.
 9. The beam delivery system; according to claim8, wherein angles of inclination of the projections are differentbetween adjacent projections.
 10. The beam delivery system according toclaim 8, wherein angles of inclination of the projections andindentations are randomly distributed.
 11. The beam delivery systemaccording to claim 7, wherein the amplitudes of the projections andindentations are randomly distributed.
 12. A beam delivery system,comprising: a laser for generating a beam of laser light from aplurality of longitudinal laser modes in a cavity of the laser; anoptical delay apparatus disposed in a beam path of the beam of laserlight, wherein the optical delay apparatus comprises plural reflectivesurfaces arranged such that the beam path comprises a closed loop; atleast one phase changing element comprising a phase changing surfacedisposed in the beam path of the closed loop; and a surface wavegenerator for generating surface acoustic waves propagating across asurface portion of the phase changing surface exposed to the beam oflaser light traversing the optical delay apparatus.
 13. The beamdelivery system according to claim 2, wherein the phase changing surfaceof the phase changing element is traversed by the beam of laser lighttraversing the optical delay apparatus.
 14. The beam delivery systemaccording to claim 2, wherein the phase changing surface of the phasechanging element provides one of the plural reflective surfaces of theoptical delay apparatus.
 15. The beam delivery system according to claim2, wherein the optical delay apparatus comprises a semi-reflectivemirror traversed by the beam of laser light traversing the optical delayapparatus.
 16. The beam delivery system according to claim 15, wherein aportion of the beam of laser light traversing the optical delayapparatus traverses the semi-reflective mirror to coincide with a beamof laser light reflected from the semi-reflective mirror.
 17. A beamdelivery system, comprising: a laser for generating a beam of laserlight from a plurality of longitudinal laser modes in a cavity of thelaser; an optical delay apparatus disposed in a beam path of the beam oflaser light, wherein the optical delay apparatus comprises pluralreflective surfaces arranged such that the beam path comprises a closedloop; at least one phase changing element disposed in the beam path ofthe closed loop, wherein the phase changing element comprises astructured phase changing surface having a plurality of projections andindentations, and wherein the optical delay element includes a prismmade of a CaF₂ material having a crystal orientation such that a (100)crystal plane is oriented under an angle of 45° relative to a surface ofthe prism.
 18. A projection exposure system for imaging a patterningstructure onto a substrate, the system comprising: the beam deliverysystem according to one claim 2; a projection optical system for imagingan object plane into an image plane; a first mount for mounting thepatterning structure in a region of the object plane within a beam pathof the second beam of laser light generated by the beam delivery system;and a second mount for mounting the substrate in a region of the imageplane of the projection optical system.
 19. A projection exposure systemfor imaging a patterning structure onto a substrate, the systemcomprising: the beam delivery system according to one claim 7; aprojection optical system for imaging an object plane into an imageplane; a first mount for mounting the patterning structure in a regionof the object plane within a beam path of the second beam of laser lightgenerated by the beam delivery system; and a second mount for mountingthe substrate in a region of the image plane of the projection opticalsystem.
 20. A projection exposure system for imaging a patterningstructure onto a substrate, the system comprising: the beam deliverysystem according to one claim 12; a projection optical system forimaging an object plane into an image plane; a first mount for mountingthe patterning structure in a region of the object plane within a beampath of the second beam of laser light generated by the beam deliverysystem; and a second mount for mounting the substrate in a region of theimage plane of the projection optical system.
 21. A projection exposuresystem for imaging a patterning structure onto a substrate, the systemcomprising: the beam delivery system according to one claim 17; aprojection optical system for imaging an object plane into an imageplane; a first mount for mounting the patterning structure in a regionof the object plane within a beam path of the second beam of laser lightgenerated by the beam delivery system; and a second mount for mountingthe substrate in a region of the image plane of the projection opticalsystem.